Clownfish to the left of me, goatfish to the right, here I am, stuck in the middle of a stunning coral reef which may not survive.

I’m snorkelling off Lady Elliott Island, a coral cay at the southern tip of the Great Barrier Reef. Below are handbag-sized clams with luminescent green lips and black sea slugs frosted with sugary sand. Every imaginable dance move is going on around me – flicking, swaying, shimmying, darting – a hyperactive ballet troupe, except the dancers are all fish.

Come to think of it, this reef has all the art forms covered: the Rembrandt blue starfish, the Ballets Russes damselfish, the Stravinsky-esque percussion of a million tiny mouths nibbling on coral.

Even football gets a look-in. I nickname a little black and white guy the Collingwoodfish (it’s actually called a humbug). Later I will discover there is another one (I kid you not) called the Chinese footballer cod.

Back on the beach I watch a woman from our tour group trying to cajole her young son into snorkelling. He’s shaking his head and whimpering. There’s scratchy sand in his fins. “Don’t you realise,” I want to whisper urgently to him, “this could be your last chance!”

Mass bleaching has affected two thirds of the Great Barrier Reef in the last couple of years, killing huge quantities of coral. Climate change is a major factor. The southern part, including Lady Elliott Island, has sustained less damage than the north. But a local ranger tells us there was worrying bleaching here last summer, halted only by the cooler currents that came with Cyclone Debbie.

Later our group bobs above the coral in a glass-bottomed boat. The boy who wouldn’t go snorkelling leans over the glass, spotting the fish and turtles mooching below us. With the encouragement of the ranger he pulls his goggles on and slides into the water.

Half an hour later the boy hauls himself back onto the boat, teeth chattering, eyes wide with the shock of pleasure. ‘I saw a manta ray!’ he tells us. ‘And Nemo!’

The Germans probably have a word for the grief you feel when you’ve not yet lost something, but suspect you soon will. As we fly back to the mainland over the endless blue, there are whales to the left of us, dolphins to the right. Here we are, stuck in the middle of paradise, and we’re running out of time.